Transcriptional regulation of development and homeostasis in complex eukaryotes, including humans and other mammals, birds, and fish, is controlled by a wide variety of regulatory substances, including steroid and thyroid hormones. These hormones exert potent effects on development and differentiation in phylogenetically diverse organisms and their actions are mediated as a consequence of their interactions with specific, high affinity binding proteins referred to as receptors. See generally, Jensen, et al., (1972); Gorski, et al., (1976); Yamamoto, et al., (1976); O'Malley, et al., (1969); Hayward, et al., (1982); and Asburner, et al., (1978).
Receptor proteins, each especially specific for one of the several classes of cognate steroid hormones (i.e., estrogens (estrogen receptor), progestogens (progesterone receptor), glucocorticoids (glucocorticoid receptor), androgens (androgen receptor), aldosterones (mineralocorticoid receptor) or for cognate thyroid hormones (thyroid hormone receptor), are known and distributed in a tissue specific fashion. See Horwitz, et al., (1978) and Pamiter, et al., (1976).
Turning now to the interaction of hormones and receptors, it is known that a steroid or thyroid hormone enters cells by facilitated diffusion and binds to its specific receptor protein, initiating an allosteric alteration of the protein. As a result of this alteration, the hormone/receptor complex is capable of binding to certain specific sites on chromatin with high affinity. See Yamamoto, et al., (1972) and Jensen, et al., (1968).
It is also known that many of the primary effects of steroid and thyroid hormones involve increased transcription of a subset of genes in specific cell types. See Peterkofsky, et al., (1968) and McKnight, et al., (1968). Moreover, there is evidence that activation of transcription (and, consequently, increased expression) of genes which are responsive to steroid and thyroid hormones (through interaction of chromatin with hormone receptor/hormone complex) is effected through binding of the complex to enhancers associated with the genes. (See Khoury, et al., 1983.)
In any case, a number of steroid hormone and thyroid hormone responsive transcriptional control units, some of which have been shown to include enhancers, have been identified. These include the mouse mammary tumor virus 5′-long terminal repeat (MTV LTR), responsive to glucocorticoid, aldosterone and androgen hormones; the transcriptional control units for mammalian growth hormone genes, responsive to glucocorticoids, estrogens, and thyroid hormones; the transcriptional control units for mammalian prolactin genes and progesterone receptor genes, responsive to estrogens; the transcriptional control units for avian ovalbumin genes, responsive to progesterones; mammalian metallothionein gene transcriptional control units, responsive to glucocorticoids; and mammalian hepatic alpha2u-globulin gene transcriptional control units, responsive to androgens, estrogens, thyroid hormones and glucocorticoids. (See the Introduction portion of Experimental Section I of this Specification for references.)
A major obstacle to further understanding and more practical use of the steroid and thyroid hormone receptors has been the lack of availability of the receptor proteins, in sufficient quantity and sufficiently pure form, to allow them to be adequately characterized. The same is true for the DNA gene segments which encode them. Lack of availability of these DNA segments has prevented in vitro manipulation and in vivo expression of the receptor-coding genes, and consequently the knowledge such manipulation and expression will yield.
The present invention is directed to overcoming these problems of short supply of adequately pure receptor material and lack of DNA segments which encode the receptors.